CFD Analysis of Phase Holdup Behaviour in a Three Phase Fluidized

Bed

A Project submitted to the National Institute of Technology, Rourkela

In partial fulfillment of the requirements

of the degree of Bachelor of Technology (Chemical Engineering)

By

Allada Chidambaram Roll No. 107CH030

Under the guidance of

Dr. H. M. Jena

DEPARTMENT OF CHEMICAL ENGINEERING NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA

ORISSA -769 008, INDIA 2011

ii

CERTIFICATE

This is to certify that the thesis entitled “CFD analysis of Phase holdup behaviour in a Three

Phase Fluidized Bed”, submitted by Allada Chidambaram (107CH030) to National Institute

of Technology, Rourkela is a record of bonafide project work under my supervision and is

worthy for the partial fulfillment of the degree of Bachelor of Technology (Chemical

Engineering) of the Institute. The candidate has fulfilled all prescribed requirements and the

thesis, which is based on candidate’s own work, has not been submitted elsewhere.

Supervisor

Dr. H. M. Jena

Department of Chemical Engineering

National Institute of Technology

Rourkela - 769008

INDIA

iii

ACKNOWLEDGEMENT

With a feeling of great pleasure, I express my sincere gratitude to Prof. H.M. Jena for his

superb guidance, support and constructive criticism, which led to the improvements and

completion of this project work.

I am thankful to Prof. R.K Singh for acting as project coordinator.

I am also grateful to Prof. K C Biswal, Head of the Department, Chemical Engineering for

providing the necessary facilities for the completion of this project.

A.CHIDAMBARAM (107CH030)

4th Year

B.Tech

Department of Chemical Engineering

National Institute of Technology, Rourkela

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ABSTRACT

Fluidized beds are used extensively in various fields of engineering since they have the potential

to promote high levels of contact between gases, liquids and solids. A characteristic set of basic

properties of a fluidized bed can be practically used which is absolutely essential for the modern

process and chemical engineering. Knowledge of the onset of fluidization is highly relevant and

it serves as the key to three-phase fluidized-bed reactors design and safe operation. In a Three-

phase fluidization the particulate solid is suspended in an upward co-current flow of gas and

liquid. Three phase fluidized beds also commonly referred to as Gas-Liquid-Solid fluidized beds

are used extensively in petrochemical, refining and food processing industries. The most

important industrial application of the process is the heterogeneous catalytic hydrogenation of

residual oils or coal slurry for the removal of sulfur and the production of hydrocarbon distillates

by hydrocracking. Keeping in view the complexity of the hydrodynamic behavior of gas-liquid-

solid fluidized, CFD analysis is used to predict the various characteristics.

In the present work FLUENT 6.3.26 has been used to study co-current gas-liquid-solid

fluidization. CFD simulations have been done for a column of height 1.88 m and diameter 0.1 m

containing glass beads as solid particles of size 2.18 mm. In the present work, involving three

phase fluidized bed, CFD analysis of phase holdup behavior is studied. GAMBIT 2.3.16 has

been used to generate a 2D coarse grid. Individual phase holdup have been determined in this

present work. CFD results indicate that the gas as well as solid holdup increases with increasing

gas velocity and decreasing liquid velocity. Liquid holdup increases with increasing liquid

velocity and decreasing air velocity. The results have been compared with those calculated from

the experimental work and have been found to agree well.

Keywords: Three phase fluidized bed, phase holdup, CFD

v

CONTENTS

CERTIFICATE ii

ACKNOWLEDGEMENT iii

ABSTRACT iv

CONTENTS v

LIST OF FIGURES viii

LIST OF TABLES x

NOMENCLATURE xi

CHAPTER 1 INTRODUCTION

1.1 Gas-liquid-solid fluidized bed 1

1.2 Advantages of Gas-liquid-solid fluidized bed 2

1.3 Application of Gas-liquid-solid fluidized bed 3

1.4 Shortcomings of Gas-liquid-solid fluidized bed 4

1.5 Modes of Operation of Gas-liquid-solid fluidized bed and flow regimes 5

1.6 Variables affecting the quality of fluidization 6

1.7 Need for CFD 6

1.8 Thesis Layout 7

vi

CHAPTER 2 LITERATURE REVIEW

2.1 Scope 8

2.2 Experimental Review 8

2.3 Computational work 11

CHAPTER 3 CFD METHODOLOGIES FOR MULTIPHASE FLOW

3.1 CFD (Computational Fluid Dynamics) 14

3.2 Benefits of CFD 15

3.3 CFD process 16

3.4 Limitations of CFD 17

3.5 Comparative study of Experimental, Analytical and Numerical methods 17

3.6 Approaches to Multi-Phase Modeling 19

3.6.1 The Euler-Lagrange approach 19

3.6.2 The Euler-Euler Approach 19

3.6.2.1 The VOF Model 20

3.6.2.2 The Mixture Model 20

3.6.2.3 The Eulerian Model 21

3.7 Choosing a General Multiphase Model 21

CHAPTER 4 CFD SIMULATION OF GAS-LIQUID-SOLID FLUIDIZED BED

4.1 Computational Flow Model 22

4.1.1 Equations 22

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4.1.2 Turbulence Modeling 23

4.1.3 Phase Holdup 24

4.2 Problem description 24

4.3 Simulation 25

4.3.1 Geometry and Mesh 25

4.3.2 Selection of models for simulation 26

4.3.3 Solution 27

CHAPTER 5 RESULT AND DISCUSSION

5.1 Phase Dynamics 31

5.2 Bed Expansion 36

5.2.1 Bed Height 37

5.3 Phase Holdup 39

5.3.1 Gas Holdup 41

5.3.2 Liquid Holdup 41

5.3.3 Solid Holdup 42

CHAPTER 6 CONCLUSION 45

REFERENCES 46

viii

LIST OF FIGURES

FIGURE NO. DESCRIPTION PAGE NO.

Fig.1. Mesh Created in Gambit 25

Fig.2. Plot of Residuals for k-휀 solver method as the iteration proceeds 28

Fig.3. Contours of volume fraction of glassbeads at water velocty of 29

0.12m/s and air velocity of 0.025m/s for initial bed height of 21.3

Fig.4. Contours of volume fraction of glass beads at water velocity 30

0.12 m/s and air velocity 0.025 m/s for initial bed height of 17.1 cm.

Fig.5. Contours of volume fractions of solid, liquid and gas at a water 31

velocity of 0.12 m/s and air velocity of 0.025 m/s for initial bed

height of 21.3 cm.

Fig.6. Velocity vectors of glass beads in the column 32

Fig.7. Velocity vector of water in the column 33

Fig.8. Velocity vector of air in the column 34

Fig.9. XY plot of velocity magnitude of water 35

Fig.10. XY plot of velcoity magnitude of air 35

Fig.11. Contours of the volume fraction of glass beads with increasing water 36

velocity at inlet air velocity of 0.075 m/s for initial bed height

of 21.3 cm.

ix

Fig.12. XY plot of glass beads at an air velocity of 0.025m/s and water velocity 37

of 0.12m/s

Fig.13. Bed Expansion vs Water velocity for initial bed height of 21.3 cm 38

Fig.14. Bed Expansion vs Air Velocity for bed height of 21.3 cm 38

Fig.15. Gas holdup vs Water Velocity for initial bed height of 21.3 cm 40

Fig.16 Gas holdup vs Air Velocity for initial bed height of 21.3 cm 40

Fig.17. Liquid holdup vs Water Velocity for initial bed height of 21.3 cm 41

Fig.18. Liquid Holdup vs Air Velocity for initial bed height of 21.3 cm 41

Fig.19 Solid Holdup vs Water Velocity for initial bed height of 21.3 cm 42

Fig.20. Solid Holdup vs Air Velocity for initial bed height of 21.3 cm 43

Fig.21. Comparision of experimental result of gas holdup with that of the 43

Simulated result obained at inlet air velocities of 0.0625m/s for

initial bed height of 21.3 cm.

x

LIST OF TABLES

TABLE NO. DESCRIPTION PAGE NO.

Table 3.1 Comparison of Experimental, Analytical and Numerical 18

Methods of Solution

Table 4.1 Properties of air, water and glass beads used in the 24

experiment

Table 4.2 Model constants used for simulation 26

xi

NOMENCLATURE

g acceleration due to gravity, m sec-2

k turbulent kinetic energy, J

t time

Mi,l interphase force term for liquid phase

Mi,g interphase force term for gas phase

Mi,s interphase force term for solid

Greek Symbols

훼k volume fraction of phase k

vm mass-averaged velocity

휌m mixture density, kg m-3

휌k density of phase k = g (gas), l (liquid), kgm-3

n number of phases

휇eff effective viscosity

휇m viscosity of the mixture, Pa s

휇l liquid viscosity, Pa s

휀k volume fraction of phase k = g (gas), l (liquid), s (solid)

휀 rate of dissipation of turbulent kinetic energy, m2s-3

uk velocity of phase k = g (gas), l (liquid), s (solid)

1

CHAPTER 1

INTRODUCTION

Fluidization is a process whereby a granular material is converted from a static solid-like state to

a dynamic fluid-like state. This process occurs when a fluid (liquid or gas or both) is passed up

through the granular material. If a fluid is passed through a bed of fine particles at a low flow

rate, the fluid merely percolates through the void spaces between stationary particles. This

condition is called the fixed bed. With an increase in flow rate, particles move apart and a few

vibrate and move in restricted regions. This condition is called the expanded bed. At a still higher

velocity, a point is reached where all the particles are just suspended by upward flowing fluid. At

this point the frictional force between particle and fluid just counterbalances the weight of the

particles, the vertical component of the compressive force between adjacent particles disappears,

and the pressure drop through any section of the bed about equals the weight of fluid and

particles in that section. The bed is considered to be just fluidized and is referred to as minimum

fluidization. At this critical value, the bed is said to be fluidized and will exhibit fluidic behavior.

When fluidized, a bed of solid particles will behave as a fluid, like a liquid or gas. (Levenspiel et

al., 1991).

1.1 Gas-Liquid-Solid Fluidized Bed

In recent years gas-liquid-solid fluidizing beds have emerged as one of the most promising

devices for three phase operations. They are considered immensely important in chemical and

bio-chemical industries, treatment of waste water and other biochemical processes. Three phase

fluidization mainly refers to the fluidization of solid particles by co-current upward flow of gas

and liquid phase. It serves the purpose of bringing the three phases in contact in a single

operation. In this type of reactor, gas and liquid are passed through a granular solid material at

2

high enough velocities to suspend the solid in fluidized state. A porous plate called distributor

supports the solid particles in the fluidized state at the static condition. It is through the

distributor plate that the fluid is forced upwards through the solid material. Initially at lower fluid

velocities the solid remains in place as the fluid passes through the voids in the material.

Subsequently as the fluid velocity is increased, the bed reaches a stage where the force of the

fluid on the solids is enough to balance the weight of the solid material. This stage is minimum

fluidization and the corresponding fluid velocity is called minimum fluidization velocity. Once

this minimum velocity is surpassed, the contents of the bed begin to expand and swirl around

much like an agitated tank or boiling pot of water. The system can now be called a fluidized bed.

(Howard., 1989).

The three phase reactors can be classified as slurry bubble column reactors and fluidized bed

reactors depending upon the density and volume fraction of the particles. In a slurry bubble

column the density of the particles is slightly higher than the liquid and size of particles range

from 5-150 µm and volume fraction is below 0.15. The liquid phase along with the solid

particles can be treated as the homogeneous liquid with mixture density. However in a fluidized

bed reactors, the density of particles are much higher than the density of the fluid and particle

size is normally large than 150 µm while the volume fraction of the solid particles varies from

0.6, in case of packed stage to 0.2 as in close transport stage (Paneerselvam et al., 2009)

1.2 Advantages of Gas-Liquid-Solid Fluidized Bed

The chief advantage of fluidization are that the solid particles are vigorously agitated by the

fluid passing through the bed, and the mixing of the solid ensures that there are practically no

temperature gradients in the bed even with quite exothermic or endothermic reactions. Some of

the advantages are as follows (Heidari, 2007 and Kumar, 2009).

3

The smooth, liquid-like flow of particles allows continuous automatically controlled

operations with ease of handling.

High rate of reaction per unit reactor volume can be obtained through these reactors.

The rapid mixing of solids leads to nearly isothermal conditions throughout the reactor,

hence the operation can be controlled simply and reliably.

It is suited to large-scale operations.

The circulation of solids between two fluidized beds makes it possible to transport the

vast quantities of heat produced or needed in large reactors.

Heat and mass transfer rates between gas and particles are high when compared with

other modes of contacting.

Benefits in economic, operational and environmental terms can be achieved with

fluidized bed technology over traditional technologies.

1.3 Application of Gas-Liquid-Solid Fluidized Bed

In the last decade or so, application of fluidized bed has been important to various chemical

industrial processes such as in pharmaceutical, metallurgical and mineral processing, in heat

transfers, catalytic cracking, pyrolysis, combustion, etc. (Levenspiel et al., 1991). Fluidization

has been used in many different processes, including the production of silicon for

semiconductors and cultivation of micro-organisms in what has come to be called as

“biofluidization” (Mahmood A., 2008). The gas-liquid-solid fluidized bed has emerged as one of

the most promising devices for three phase operations Fluidized beds serve many purposes in

industry, such as in the hydrogen –oil process for hydrogenation and hydrodesulphurization of

residual oil, the H-coal process for coal liquefaction, and fischer-tropsch process (Jena et al.,

2009).

4

Some of the commercial applications are as follows:

Fluid catalytic cracking, reforming

Fischer-Tropsch synthesis

Catalyst regeneration

Granulation (growing particles)

Oxidation reactions involving solid catalyzed gas phase reactions

Hydrotreating, hydroprocessing

Biochemical processes

Fluid coking

Bio-oxidation process for waste water treatment

Drying purposes ( hot air drying of dolomite, coal grains)

In transportation of solids like slurry pipeline for coal

1.4 Shortcomings of Three Phase Fluidized Bed

It is difficult to describe flow of gas, with its large deviation from plug flow and

bypassing of solids by bubbles results in an inefficient contacting system.

The rapid mixing of solids in the bed leads to non-uniform residence time of solids in the

reactor.

For non-catalytic operations at high temperature the agglomeration and sintering of fine

particles can necessitate a lowering in temperature of operation, resulting in the reduction

of reaction rate (Lee ., 2002).

Fluid like behavior of the fine solid particles within the bed results in abrasion leading to

the erosion of pipes and vessels.

5

Attrition of solids is one of the main disadvantages. Because of attrition, the size of the

solid particles gets reduced and possibility for entrapment of solid particles with the fluid

is more.

1.5 Modes of Operation of Gas-Liquid-Solid Fluidized Bed and Flow Regimes

Several types of operation for gas-liquid-solid fluidized bed are possible based on the differences

in flow directions of gas and liquid and also based on the differences in the pattern of contact

between the particles and the surrounding gas and liquid. The three phase fluidization can be

classified into four modes of operation. These modes are as follows:

Co-current three phase fluidization with liquid as the continuous phase.

Co-current three phase fluidization with gas as the continuous phase.

Inverse three phase fluidization.

Fluidization by a turbulent contact absorber.

According to Epstein (1981), the liquid supported solid operation characterizes fluidization with

the liquid velocity beyond the minimum fluidization velocity. The bubble supported solid

operations characterizes fluidization with the velocity of liquid below the minimum fluidization

velocity and hence it may be possible that the liquid is in stationary state.

Multiphase flow regimes can be grouped into four categories:

Gas-liquid or liquid-liquid flows

Gas-solid flows

Liquid-solid flows

Three phase flows

In this present work three phase flows is of prime importance which is actually the combination

of the other flow regimes listed above it.

6

1.6 Variables Affecting the Quality of Fluidization

Variables which influence the quality of fluidization are as follows (Kumar, 2009):

Fluid flow rate – Flow rate should be high enough to keep the solids in suspension but it

should not be too much high that the fluid channeling occurs.

Fluid inlet – Inlet must be designed in such a way that there is always well distribution of

the fluid entering the bed.

Particle size – Size of the particles greatly affect the quality of fluidization. Particles of

different sizes are grouped into Geldart’s classification of particles. Particles having a

wide range are easier to fluidize than the particles of uniform size.

Gas, liquid and solid densities – Closer is the relative density of the gas-liquid and the

solid, easier is to maintain smooth fluidization.

Bed height – With other variables remaining constant, it is more difficult to obtain good

fluidization when the bed height is greater.

1.7 Need for CFD

Computational fluid dynamics is a whole new field which needs to be explored well. Over the

recent years there have been various computational works but in comparison to the huge

experimental data available, more works in the field of CFD is required. CFD predictions can be

verified with the experimental data and results and can be checked if they hold good or not. With

the experimental work being tedious, CFD helps in predicting the fluid flow, behavior of the

fluidized bed and various hydrodynamic characteristics. CFD actually helps in modeling the

prototype of a real world process and through CFD predictions one can apply those parameters to

achieve the desired results. Thus the complex hydrodynamics of fluidization could be understood

using CFD.

7

Objective of the work:

The aim of the present work could be summarized as follows:

Study of complex hydrodynamics of gas-liquid-solid fluidized bed.

Determining the individual phase holdup in a gas-liquid-solid fluidized bed.

Analysis of the phase holdup behavior and various parameters that affect it.

Examining the effect of superficial gas and liquid velocity on the individual phase

holdup.

The present work is concentrated on understanding the phase holdup behavior in a three phase

fluidized bed. Fluidized bed of height 1.88 m with diameter of 0.1 m has been simulated. Glass

beads of diameter 2.18 mm are used as the solid phase. Co-current gas-liquid-solid fluidization

has been done with the liquid (water) as the continuous phase. Liquid (water) and Gas (air) has

been injected at the base with different superficial velocities. The static bed heights of the solid

phase in the fluidized bed used for simulation are taken as 21.3 cm and 17.1 cm. In all the cases

the initial holdup is taken to be 0.59 with the superficial velocity of gas varying from 0.0125-0.1

m/s and that of liquid ranging from 0.0-0.15 m/s. CFD simulations have been carried out using

FLUENT 6.3, CFD Software. GAMBIT 2.3.16 has been used to design the Mesh.

1.8 Thesis Layout

The second chapter gives a comprehensive review of literature related to the hydrodynamic

characteristics and phase holdup behavior. It includes the experimental as well as the

computational aspect of gas-liquid-solid fluidization. The third chapter deals with the CFD

methodologies for multiphase flows where the various basic approaches to the problem solution

have been discussed. Computational flow model and the basic equations governing it have been

discussed in chapter four. Chapter five deals with the result part obtained from simulations which

have been discussed clearly. In chapter six (the last one) conclusions have been drawn on present

work and scope of the future work has been presented.

8

CHAPTER 2

LITERATURE REVIEW

2.1 Scope

The fluidized bed is one of the best known contacting methods used in processing industry.

There are many well established operations that utilize this technology. Three phase fluidized

beds have been widely used in the polymer, chemical, petrochemical and biochemical industries.

The aim of this chapter is to give a comprehensive review of literature related to the

hydrodynamic characteristics and phase hold up behavior in a gas-liquid-solid fluidized bed. An

overview of the literature relevant to this study is presented next. The first section deals with the

experimental works done and the succeeding section deals with the CFD predictions.

2.1.1 Experimental Review

A significant amount of experimental work has been done regarding the hydrodynamics and

other characteristic behavior of fluidized bed. Various aspects have been considered ranging

from the particle diameter to the fluid velocity. Earlier more emphasis was given on the

hydrodynamics of a fluidized bed which dealt with the bed voidage, bed expansion, etc but

recent studies have been done on the phase holdup, mass transfer in a fluidized bed, flow regime

identification, etc. Various works on Gas-Liquid-Solid fluidization have been discussed below:

Soung Y. (1977) described three phase fluidization as a fluidization of solid particles by co-

current upward flow of liquid and gas phase. Since it provides for contact between the phases in

a multiphase flow systems, it has the advantage of superior heat transfer characteristics. He made

an attempt to isolate the gas injection effect on the expansion of bed from the liquid viscosity. He

even presented the correlations for the effect of gas velocity on bed expansion.

9

Kim et al. (1980) studied the phase holdup for beds of solids having binary size distributions, and

correlated the phase holdup data empirically by the equations involving Reynolds and Froude

numbers based on harmonic mean particle diameter. They examined that liquid phase holdups

showed a decreasing trend with the subsequent increase in gas velocity and with decreasing

liquid velocity. However, at higher gas velocities, liquid phase holdup is nearly independent of

gas velocities, which may be due to the bubbly flow regimes progressed to intermediate or slug

flow regimes with higher gas flow rates. Gas phase holdup increased with increasing gas

velocity. Both the bed porosity and gas phase holdups increased with gas as well as liquid

velocities.

For chemical processes where mass transfer is the rate limiting step, it is important to estimate

the gas holdup, since this relates directly to the mass transfer, as concluded by Fan., (1989).

Lee and De Lasa (1987) investigated the bubble phase holdup and velocity in three phase

fluidized beds. They used various operating conditions like elctroresistivity probe and optical

fiber probe.

Muroyama and Fan, (1985) studied the behavior of a three phase fluidized bed and hence

concluded that in a typical gas–liquid–solid three-phase fluidized bed, solid particles are

fluidized primarily by upward concurrent flow of liquid and gas, with liquid as the continuous

phase and gas as dispersed bubbles if the superficial gas velocity is low. Because of the good

heat and mass transfer characteristics, three-phase fluidized beds or slurry bubble columns (ut <

0.05 m/s) have gained considerable importance in their application in physical, chemical,

petrochemical, electrochemical and biochemical processing.

Kim et al.,(1989) studied the phase holdup characteristics and bed porosity in three phase

fluidized beds with a wide range of liquid properties and determined the effect of liquid and gas

10

velocities, particle size, liquid properties such as viscosity and surface tension on the individual

phase holdup and bed porosity in three phase fluidized beds. They concluded that liquid phase

holdup increases with liquid velocity and viscosity, column diameter and liquid surface tension.

It decreases with gas velocity, particle size and density difference between solid and liquid

phases.

Krisnaiah et al. (1993) conducted experiments to study the hydrodynamics of three phase inverse

fluidized beds using very light particles. Experimental data relating the minimum liquid velocity

at the onset of fluidization was correlated in terms of particle characteristics, physical properties

of fluids and system variables. He even proposed correlations for the friction factor.

Comte et al. (1997) studied the complex hydrodynamics of a three phase inverse turbulent bed

and discussed about the critical gas velocity required to distribute the particles over the whole

height of the reactor. They also discussed about the gas velocity required for uniform axial

distribution of the solids.

After a thorough study on the hydrodynamics of gas-liquid-solid fluidized bed bio-reactor with a

low density biomass support, Sokol and Halfani(1999) concluded that air holdup increases with

increase in the inlet air velocity.

sivakumar et al. (2008) examined that the larger drag forces applied to the solid particles by an

increase in liquid velocity cause an increase in liquid holdup. However for a constant liquid

velocity, variation in liquid holdup with an increase of gas velocity was not significant. Liquid

holdup decreases with an increase in particle diameter. Increase of particle sphericity reduces the

surface area of particle per unit volume which leads more bubble breakage and hence liquid

holdup decreases. At a constant fluid velocity, increase in liquid viscosity causes higher drag

forces acting on the solid particles resulting in an increase in liquid holdup. Increasing liquid

11

viscosity/fluid consistency index enhances the liquid interface and hence an increase in liquid

holdup.

Schubert et al. (2009) worked on the phase holdups in three phase semi-fluidized bed and

examined that the liquid saturation in the fixed bed increases with increasing superficial liquid

velocity. For the same gas and superficial liquid velocities, the liquid saturation decreases with

decrease in liquid kinematic viscosities. On the other hand opposite results were found for the

gas saturation. The gas holdup in the fluidized bed is identified to be the most important and

critical parameter.

2.1.2 Review of Computational Work

Computational Fluid Dynamics (CFD) that have been popular for quite a while in other fields of

science (like space and mechanical engineering) and are becoming more and more interesting for

chemical engineers. Experiments made way into computational analysis since experiments were

tedious and time taking to perform. In computational fluid dynamics, advanced modeling

approaches based on CFD techniques were applied for investigation of three phases in order to

achieve accurate design and scale up. Some of the works are discussed below:

Bahary et al. (1994) used Eulerian multi-fluid approach for three-phase fluidized bed, where gas

phase was treated as particulate phase having 4 mm diameter and a kinetic theory granular flow

model was applied for the solid phase. They simulated both symmetric and axisymmetric model

and verified the various flow conditions in the fluidized bed by comparing with the experimental

data.

Schuteze et al. (1998) focused on CFD modeling of oxygen transfer in a stirred tank bioreactor

and succeeded in even including the free surface into their mass transfer calculations.

CFD modeling results using CFX-4.2 for a randomly packed distillation column were presented

by Yin et al. (2000). They compared their results to measurement data obtained at different

12

Operating conditions from a 1.22-m diameter, 3.66 m high packed bed that was equipped with

several sizes of Pall rings. Good agreement with the predictions was achieved giving rise to the

hope that CFD will become a useful tool for the scale-up of this class of apparatus as well.

Similar computations have been carried out for semi-structured catalytic packed beds by Calis et

al. (2001) using CFX-5.3. They found that pressure drop in such beds can be predicted with an

error of less than 10 % compared to measurement results; still for such precision, very fine

discretization grids (up to three million grid cells) and correspondingly high computational

power is necessary.

Bauer and Eigenberger (2001) in their research paper called “Multiscale approach” described the

fluid dynamics via CFD computation and included chemical reaction and mass transfer by means

of a zone model.

Rusche (2002) investigated about the bubbles rising in a stagnant fluid and in a shear flow. A

two-fluid (Euler-Euler) methodology was adapted to high phase fractions. Experimental data was

used to verify the computational results. He concluded that computational results for air bubbles

rising in quiescent water showed features like path instability and the shedding of wakes.

Basically two types of trajectories were observed depending upon the bubble size. Bubbles with

nominal diameter larger than 2mm prescribed a helical trajectory whereas smaller bubbles

exhibit zigzag motion.

Joshi et al. (2002) determined the prediction of flow pattern near the wall and pressure drop in a

bubble column reactor by using k-ɛ based model with low Reynolds number. Modeling of

momentum transfer near the wall has been given special attention. The predicted and

experimental hold-up and velocity profiles over wide range of column diameter, column height,

bubble diameter and bubble rise show an excellent agreement.

Schallenberg et al. (2005) used 3-D, multi fluid Eulerian approach for three phase bubble

column. Extended k-ɛ turbulence model were used to account for bubble induced turbulence and

13

intherphase momentum between two dispersed phases were included. They validated the local

gas and solid holdup as well as liquid velocities with the experimental data.

Paneerselvam et al. (2009) worked upon the CFD simulation of hydrodynamics of gas-liquid-

solid fluidized bed reactor by developing a three dimensional transient model and compared the

CFD simulation prediction with the experimental data of Kiared et al. (1999) which shows a

good agreement. They also studied the influence of different interphase drag models for gas-

liquid interaction on gas holdup.

Sivaguru et al. (2009) used CFD analysis to study the hydrodynamic characteristics of three

phase fluidized bed. They developed a two dimensional simulation using porous zone and porous

jump model. They found out that for uniform air-water flowrate, porous jump model is better

than porous zone model. As the gas flowrate is increased with constant liquid flow rate, the

pressure drop of the column decreases which is due to the reduced density in the bed since more

gas holdup is there in the bed.

From the above literature review it is found that a lot of experimental work has been done in

fluidization. Based upon the experimental results, many computational works have been verified.

However many computational works are related in understanding the hydrodynamic behavior of

a fluidized bed and very few are dedicated to the phase holdup analysis in three phase fluidized

bed. From the previous studies it can be concluded that not much emphasis has been given on the

phase holdup behavior. Hence an attempt is made in this present work to focus on the phase

holdup behavior and to understand and completely analyze the phase holdup in a fluidized bed

and also the various parameters upon which it depends.

14

CHAPTER 3

CFD METHODOLOGIES FOR MULTIPHASE FLOW

Multiphase flow is simultaneous flow of

Materials with different states or phases (i.e. gas, liquid or solid).

Materials with different chemical properties but in the same state or phase (i.e. liquid-

liquid systems such as oil droplets in water).

Thus Multiphase flow refers to a situation where two or more fluids are present. Each fluid may

have its own flow field, or all fluids may share a common flow field. One of the phases is

continuous (primary) while the others (secondary) are dispersed within the continuous phase. A

diameter has to be assigned for each secondary phase to calculate its interaction (drag) with the

primary phase. A secondary phase with a particles size distribution is modeled by assigning a

separate phase for each particle diameter. Each phase can be laminar or turbulent. Fluid flow

(primary phase) may be turbulent with respect to the secondary phase but may be laminar with

respect to the vessel (Bakker., 2002).

Multiphase flow is important in many industrial processes:

Riser reactors

Fluidized bed reactors

Bubble column reactors

Scrubbers, dryers, etc.

Typical objectives of a modeling analysis:

Maximize the contact between the different phases, typically different chemical

compounds.

Flow dynamics.

15

This is where CFD (Computational Fluid Dynamics) comes into the picture in order to achieve

the above said objectives of a multiphase flow.

3.1 CFD (Computational Fluid Dynamics)

CFD is a technology that enables us to study the dynamics of things that flow. Using CFD we

can develop a computational model of the system or device that we want to study. Fluid flow

physics is applied along with some chemistry and the software will output the fluid dynamics

and the related physical phenomena. With the advent of CFD, one has the power to simulate the

flow of gases and liquids, heat and mass transfer moving bodies, multiphase physics, chemical

reaction, fluid structure interaction and acoustics through computer modeling. A virtual

prototype of the system can be built using CFD (Fluent Inc, 2001). Thus CFD can applied for

predicting the fluid flow associated with the complications of simultaneous flow of heat, mass

transfer, phase change, chemical reaction, etc. using computers (Chaitanya .,2003). CFD has now

become an integral part of the engineering design and analysis. Engineers can make optimal use

of the CFD tools to simulate fluid flow and heat transfer phenomena in a system model and can

even predict the system performance before actually manufacturing it (Ghoshdastidar., 1998).

3.2 Benefits of CFD

Insight- if there is a device or system design which is difficult to analyze or test through

experimentation, CFD analysis enables us to virtually sneak inside the design and see

how it performs. CFD gives a deep perception into the designs. There are many

occurrences that we can witness through CFD which wouldn’t be visible through any

other means.

16

Foresight- under a given set of circumstances, we can envisage through the CFD software

what will happen. In a short time we can predict how the design will perform and test

many variants until we arrive at an ideal result.

Efficiency- the foresight we gain helps us to design better to achieve good results. CFD is

a device for compressing the design and development cycle allowing for rapid

prototyping.

Advantages of CFD can be summarized as (Ghoshdastidar ., 1998).

The effect of various parameters and variables on the behavior of the system can be

studied instantaneously since the speed of computing is very high. To study the same in

an experimental setup is not only difficult and tedious but also sometimes may be

impossible.

In terms of cost factor, CFD analysis will be much cheaper than setting up experiments or

building sample model of physical systems.

Numerical modeling is flexible in nature. Problems with different level of complexity can

be simulated.

It allows models and physical understanding of the problem to be improved, very much

similar to conducting experiments.

In some cases it may be the only practicable ancillary for experiments.

3.3 CFD Process

The steps underlying the CFD process are as follows:

Geometry of the problem is defined.

Volume occupied by fluid is divided into discrete cells.

Physical modeling is well-defined.

17

Boundary conditions are defined which involves specifying the fluid behavior and

properties at the boundaries.

Equations are solved iteratively as steady state or transient state.

Analysis and visualization of resulting solution is carried out.

3.4 Limitations of CFD

Even if there are many advantages of CFD, there are few shortcomings of it as follows (Bakker.,

2002)

CFD solutions rely upon physical models of real world processes.

Solving equations on a computer invariably introduces numerical errors.

Truncation errors due to approximation in the numerical models.

Round-off errors due to finite word size available on the computer.

The accuracy of the CFD solution depends heavily upon the initial or boundary

conditions provided to numerical model.

3.5 Comparative Study of Experimental, Analytical and Numerical Methods

Experimental Method - experimental methods are used to obtain consistent information about

physical processes which are not clearly understood. It is the most realistic approach for problem

solving. It may involve full scale, small scale or blown up scale model. However disadvantages

are high cost, measurement difficulties and probe errors.

Analytical Method - these methods are used to obtain solution of a mathematical model which

consists of a set of differential equations that represent a physical process within the limit of

conventions made. The systematic solution often contain infinite series, special functions etc.

and hence their numerical evaluation becomes difficult to handle.

18

Numerical method – numerical prediction works on the results of the mathematical model. The

solution is obtained for variables at distinct grid points within the computational field. It provides

for greater handling of complex geometry and non-linearity in governing equations or boundary

conditions. The kind of ease provided by numerical methods makes it the powerful and widely

applicable. The above said discussion is represented in tabular form in table 3.1

Table 3.1. Comparison of Experimental, Analytical and Numerical Methods of Solution

(Ghoshdastidar ., 1998)

Name of the Method Advantages Disadvantages

1. Experimental

Capable of being most realistic

Equipment required Scaling problem Measurement

difficulties Probe errors High operating costs

2. Analytical

Clean, general information which is usually in formula form

Restricted to simple geometry and physics

Usually restricted to linear problems

Cumbersome results-difficult to compute

3. Numerical

No restriction to linearity. Ability to handle irregular geometry and complicate physics. Low cost and high speed of computation.

Truncation and round-off errors

Boundary condition problems

19

An assessment of advantages and disadvantages of numerical methods vis-à-vis analytical and

experimental method shows that even though the Numerical Method has few shortcomings but it

has many advantages associated with it and is hence suited.

3.6 Approaches to Multiphase Modeling

The first step in solving any Multiphase problem is the selection of model. For a Multiphase flow

there are two approaches for numerical calculations:

1. Euler-Lagrange approach

2. Euler-Euler approach

3.6.1 The Euler Lagrange Approach

This kind of approach is followed by the lagrangian discrete phase model. In this approach the

fluid phase is treated as a continuous extent by solving the time averaged Navier-Stokes

equations, while the dispersed phase is solved by computing a large number of particles, bubbles

or droplets through the calculated flow-field. An altercation of momentum, mass and energy can

take place between the dispersed phase and the fluid phase. Particle trajectories are computed

individually at specified intervals during the fluid phase calculations and dispersed second phase

occupies a low volume fraction which are the fundamental assumptions in this model. These

assumptions make the model appropriate for spray dryers, coal and liquid fuel combustion and

some particle laden flows but inappropriate for the modeling of liquid-liquid mixtures, fluidized

beds or any application where volume fraction of second phase is negligible (Mohapatra and

Rakh, 2007).

3.6.2 The Euler-Euler Approach

In this approach the concept of volume fraction is introduced based upon the fact that the volume

of a phase cannot be carried or occupied by other phases. Here the different phases are treated

20

mathematically as interpenetrating continua. The volume fractions are assumed to be continuous

functions of space and time and their sum is equal to one. For each of the phase, conservation

equations are derived to obtain a set of equations which have similar structure for all phases

(Kumar, 2009).

The three different Euler-Euler Multiphase models available are

The Volume of fluid (VOF) model

The Mixture Model

The Eulerian model

3.6.2.1 The VOF Model

The VOF model can model two or more immiscible fluids by solving a single set of momentum

equations and tracking the volume fraction of each of the fluids throughout the domain. It is

generally used to figure out a time dependent solution but for problems which are concerned

with steady state solution, it is possible to perform a steady state calculation. A steady state VOF

calculation is practical only when the solution is independent of the initial conditions and there

are distinct inflow boundaries for the individual phases (Heidari, 2007)

3.6.2.2 The Mixture Model

The mixture model is designed for two or more phases (fluid or particulate). It is simplified

multiphase model that can be used to model multiphase flows where the phases move at different

velocities, but assume local equilibrium over short spatial length scales. It can also be used to

model homogenous multiphase flows with very strong coupling and the phases moving at the

same velocity. It is a good substitute for the full Eulerian multiphase model in several cases. It

can model n phases by solving the momentum, continuity and energy equations for the mixture,

21

the volume fraction equations for secondary phases and algebraic expression for the relative

velocities (Heidari, 2007)

3.6.2.3 The Eulerian Model

The Eulerian model is the most complex among other multiphase models. It solves a set of n

momentum and continuity equations for each phase coupling is achieved through the pressure

and interphase exchange coefficients. The manner of handling of these couplings depends upon

the type of phases involved. Granular (fluid-solid) flows are handled differently than non-

granular (fluid-fluid) flows. For granular flows, the properties are obtained by applying kinetic

theory (Kumar, 2009).

3.7 Choosing a General Multiphase Model

The first step in solving any problem is to determine which of the regimes provides some broad

guidelines for determining the degree of interphase coupling for flows involving bubbles,

droplets, or particles and the appropriate model for different amounts of coupling.

The appropriate model for flows involving bubbles particles or droplets are as follows (Heidari,

2007).

For bubble, droplet and particle-laden flows in which dispersed-phase volume

fractions are less than or equal to 10% the discrete phase model is to be used

For bubble, droplet and particle-laden flows in which the phases mix and / or

dispersed phase volume fractions exceed 10% the mixture model is used.

For slug flow, the VOF model is used.

For stratified / free-surface flows, VOF model is used.

For fluidized bed, Eulerian Model for granular flow is used.

For slurry flows and hydro transport, Eulerian or Mixture model is used.

22

CHAPTER 4

CFD SIMULATION OF GAS-LIQUID-SOLID FLUIDIZED BED

4.1 Computational Flow Model

Eulerian multi-fluid model is adopted in the present work where gas and liquid phases are all

treated as continuous, inter-penetrating and interacting everywhere within the computational

domain. The pressure filed is assumed to be shared by all the three phases proportional to their

volume fraction. The motion of each phase is governed by the respective mass and momentum

equations. Heidari (2007) discussed that Eulerian multiphase model in FLUENT allows for the

modeling of multiple distinct yet interacting phases. With the Eulerian multiphase model, the

number of secondary phases is limited only by memory requirements and convergence behavior.

Any number of secondary phases can be modeled, if sufficient memory is available.

4.1.1 Equations

Continuity Equation:

(휌m) + ∇. (휌m vm) = 0

Where vm is the mass-averaged velocity

vm =( ∑ 훼k 휌k vk ) / 휌m

휌m is the mixture density:

휌m = ∑ 훼k 휌k

훼k is the volume fraction of phase k

Momentum Equation:

For liquid phase:

23

(휌l휀lul) + ∇. (휌l휀l ul ul) = −휀l∇푝 + ∇. (휀l휇eff,l ( ∇푢l + ∇푢lT)] + 휌l휀l g + Mi.l

For gas phase:

(휌g휀gug) + ∇. (휌g휀g ug ug) = −휀g∇푝 + ∇. (휀g휇eff,g ( ∇푢g + ∇푢gT)] + 휌g휀g g + Mi,g

For solid phase:

(휌s휀sus) + ∇. (휌s휀s us us) = −휀s∇푝 + ∇. (휀s휇eff,s ( ∇푢s + ∇푢sT)] + 휌s휀s g + Mi,s

Where 휇eff is the number of phases, Mi.l, Mi,g, Mi,s represent the interphase force term for

liquid, gas and solid phase respectively.

4.1.2 Turbulence Modeling

The choice of turbulence model will depend upon (Heidari, 2007):

Physics encompassed in the flow

The established practice for a specific class of problem

The level of accuracy required

The available computational resources

Amount of time available for simulation

To make the most appropriate choice of model one must understand the capabilities and

limitations of the various options. In the present work standard k-휀 model is chosen for modeling

the turbulence. k-휀 is a turbulence model on the basis of RANS (Reynolds-Average Navier-

Stokes) approach and has been developed by Launder and Spalding in 1972. The turbulence

model is a set of equations which expresses relations between unknown terms appearing in the

Reynolds-averaged governing quantities. The k-휀model focuses on the mechanisms that affect

the turbulent kinetic energy and is nearly homogeneous. In the derivation of k-휀 model, the

assumption is that the flow is fully turbulent and the effects of molecular viscosity are negligible

24

(Spalding et al., 1972). Analytical expressions are used to bridge the wall boundary and the fully

turbulent fluid since the choice of the k-휀 standard wall function determines that the viscosity

affecting the near wall region is not resolved. The expression implemented in FLUENT is the

logarithmic law of the wall for velocity. Wall functions avoid the turbulence model adaptation to

the presence of the wall, saving computational resources. (Kumar, 2009).

4.1.3 Phase Holdup

The phase holdup is the fraction of individual phase occupied in three phase contacting and is

one of the most important parameter to determine the hydrodynamic properties of gas-liquid-

solid fluidized beds (Mahmood, 2008).

Since the sum total of the entire individual phase holdup is equal to one, it can be stated as:

휀l + 휀s + 휀g = 1 , where

휀l is the holdup of continuous (water)

휀s is the holdup of solid (particles)

휀g is the holdup of dispersed (air)

4.2 Problem Description

The problem comprises a three phase fluidized bed in which air and water enters at the bottom of

the column. The bed consists of solid material (glass beads) of uniform diameter which is filled

in the column up to a preferred height. The following table 4.1 shows the properties of air, water

and glass beads used.

Table 4.1 Properties of air, water and glass beads used in the experiment

Phases Density Viscosity

Air 1.225 kg/m3 1.789*10-05 kg/m-s

Water 998.2 kg/m3 0.001003 kg/m-s

Glass beads 2470 kg/m3 0.001003 kg/m-s

25

4.3 Simulation

4.3.1 Geometry and Mesh

Gambit 2.3.16 was used for making 2D

rectangular geometry with width of 0.1 m

and height 1.88 m. Coarse mesh size of 0.01

m was taken in order to have 1880 cells

(3958 faces) for the whole geometry.

Smooth mesh can be created taking mesh

size of 0.005 m. But in case of smooth mesh

the iterations per time step increases, hence

a coarse grid mesh is preferred.

Fig.1. Mesh Created in Gambit

26

4.3.2 Selection of Models for Simulation

FLUENT 6.3.26 was used for simulation. 2D segregated 1st order implicit unsteady solver is

used. Standard k-휀 dispersed Eulerian multiphase model with standard wall functions were used

for modeling turbulence. The value of various model constants is tabulated as:

Table 4.2 Model constants used for simulation

Model Constants Value

Cmu 0.09

C1-Epsilon 1.44

C2-Epsilon 1.92

C3-Epsilon 1.3

TKE Prandtl Nummber 1

TDR Prandtl number 1.3

Dispersion Prandtl Number 0.75

Water is taken as the primary phase which is the continuous phase while glass beads and air as

dispersed phase. Inter-phase interactions formulations used for Drag Coefficient were

Air-Water: Schiller-Naumann

Glass beads-Water: Gidaspow

Glass beads- Air: Gidaspow

Air velocities ranging from 0.0125 m/s to 0.1 m/s with increment of 0.0125 and water velocities

from 0 to 0.15 m/s with increment of 0.03 were used respectively. The inlet air volume fraction

was obtained as the fraction of air entering in the mixture of gas and liquid.

Pressure outlet boundary conditions:

27

Mixture Gauge Pressure- 0 Pascal

Solid and Liquid Boundary Conditions:

Backflow granular temperature- 0.0001 m2/s2

Backflow Volume Fraction- 0

Specified Shear Stress was set as X=0 and Y=0 for gas and solid whereas for water no slip

condition was used.

4.3.3 Solution

The under relaxation factor for solution control in different flow quantities were taken as

pressure = 0.3, Density = 1, Body Forces = 1, Momentum = 0.3, Volume Fraction = 0.5,

Granular Temperature = 0.2, turbulent Kinetic Energy = 0.8, Turbulent Dissipation Rate = 0.8,

Turbulent Viscosity = 1. Pressure- Velocity Coupling was chosen as Phase Coupled SIMPLE.

First Order Upwind was chosen for Discretization. The solution has been initialized from all

zones. For patching a solid volume fraction, volume fraction of the solid in the part of the

column up to which the glass beads were fed, was used. Iterations were carried out for time step

size of 0.01-0.001 liable on the ease of convergence and the time required to achieve the

fluidization results. The following figure shows the residual plot for k-휀 solver method as the

iteration proceeds.

28

Fig.2 Plot of Residuals for k-휀 solver method as the iteration proceeds

A convergence criterion of 0.001 has been used in the simulation. While simulating, the profile

of the bed changes. For a longer physical flow time, the simulations are carried out till the

solution reaches a Quasi-steady state.

29

CHAPTER 5

RESULT AND DISCUSSION A gas-liquid-solid fluidized bed of diameter 0.1m and height 1.88 m has been simulated using

commercial CFD software package FLUENT 6.3.26. Static bed of heights 17.1 cm and 21.3 cm

has been used for simulation. Diameter of the glass beads (solid phase) were taken to be 2.18

mm. Inlet superficial velocity of gas were taken in a range of 0.0125m/s to 0.1m/s while that of

water were taken in the range of 0 to 0.15 m/s. The simulation results obtained have been shown

graphically in the following figure 3.

0 0.5 1 2 4 6 8 10 12 15 18 21 25 30 35 40

sec sec sec sec sec sec sec sec sec sec sec sec sec sec sec sec

Fig.3. Contours of volume fraction of glassbeads at water velocty of 0.12m/s and air velocity of

0.025m/s for initial bed height of 21.3 cm.

30

While simulation takes place, a change in profile is seen in the column, but after some time no

significant change is observed which indicates that the quasi steady state has been reached.

Simulations were carried out till there was no change in the bed profile. From the figure it is very

much clear that the bed profile changes for the first 25 sec, after which there is no subsequent

change in the bed profile even though the simulation goes on, like that between 25 to 40 seconds.

0 0.5 1 2.5 5 7.5 10 15 18 21 23 25

sec sec sec sec sec sec sec sec sec sec sec sec

Fig.4. Contours of volume fraction of glass beads at water velocity 0.12 m/s and air velocity

0.025 m/s for initial bed height of 17.1 cm.

Here the quasi steady state is reached in the first 25 seconds.

31

5.1 Phase Dynamics

Solid, liquid and gas phase dynamics have been represented in the form of contours, vectors and

XY Plot. Figure 4 shows the contours of volume fraction of solid, liquid and gas in the column

obtained at water velocity of 0.12m/s and air velocity of 0.025m/s for static bed height of 21.3

cm after the quasi steady state is achieved. The colour scale given to the left of each contour

indicates the value of volume fraction corresponding to the colour.

Fig.5. Contours of volume fractions of solid, liquid and gas at a water velocity of 0.12 m/s and

air velocity of 0.025 m/s for initial bed height of 21.3 cm.

The contour for glassbeads shows that the bed is in fluidised condition. Contour for water shows

that volume fraction of water is less in fluidized part of the column in comparion to the rest part

of the column. The gas holdup is more in fluidzed part of the bed as illustrated by the contours

for air.

32

Vectors of velocity magnitude of glass beads, water and air in the column obtained at inlet water

velocity of 0.12m/s and inlet air velocity of 0.0125m/s for static bed height 21.3 cm are show in

the following figures.

Fig.6. Velocity vectors of glass beads in the column, (actual view and magnified view)

It is very much evident from the picture that the velocity at the bottom is small. At the top

expanded part of the bed the velocity vectors show a downward trend and since no glassbeads

are present at the top, no velocity vectors are seen. At the middle portion the direction of velocity

of the particles show downward trend but that of the particles away from the wall is vertically

upwards.

33

Fig.7. Velocity vector of water in the column (actual view and magnified view)

Velocity vector of water shown an upward trend throughout. However velocity is more in the

fluidized part of the column compared to the part where there is no glass beads. The reason is

that the flow of liquid faces an obstruction in the fluidized section of the bed where glassbeads

are present.

The following figure 8 shows the velocity vector of air. It is evident from the figure that the

velocity of air is very small in the fluidized part of the column.

34

The reason is that volume fraction of air is very less compared to that of glass beads and water in

the fluidized part of the column. Moreover the flow of air bubbles is restricted by the glass beads

present in the bed.

Fig.8. Velocity vector of air in the column (actual view and magnified view)

In the upward section of the column, it may also be possible that the high water velocity carries

the air bubble along with them, so their velocity decreases.

35

Following figure 9 and figure 10 show the XY Plot of velocity magnitudes of air and water

obtained at an inlet air velocity of 0.025m/s and water velocity 0.12m/s. A parabolic curve is

obtained in the plot which a necessary pattern for a fully developed flow.

Fig.9. XY plot of velocity magnitude of water

Fig.10. XY plot of velcoity magnitude of air.

36

5.2 Bed Expansion

The following figure shows the contours of the volume fraction of glassbeads at an inlet air

velocity of 0.075 m/s and different water velocities for static bed height of 21.3 cm and glass

beads of size 2.18 mm. The figure clearly shows that the bed epands as liquid velocity is

increased at constant gas velocity.

Water velocity 0.0 0.03 0.06 0.09 0.12 0.15

(m/s)

Fig.11. Contours of the volume fraction of glass beads with increasing water velocity at inlet air

velocity of 0.075 m/s for initial bed height of 21.3 cm.

Thus the figure shows that bed expands when liquid velocity is increased at constant air velocity.

At higher velocity of water the bed expansion is vigorous as illustrated by the above figure.

37

5.2.1 Bed Height

Bed height is determined from the X-Y plot of volume fraction of glass beads on Y-axis and

height of the column on X-axis.

Fig.12. XY plot of glass beads at an air velocity of 0.025m/s and water velocity of 0.12m/s

Even from the XY plot, one can predict the bed height observing at the Xaxis (Y-coordinate

mixture) and also the volume fraction of the glassbeads corresponding to the height shown in Y-

axis.

The variation in the nature of bed height at varying water and air velocity have been shown in the

following figures.

38

Fig.13. Bed Expansion vs Water velocity for initial bed height of 21.3 cm

The above figure illustrates that bed height shows an increasing trend as the water velocity is

increased for a particular air velocity.

Fig.14. Bed Expansion vs Air Velocity for bed height of 21.3 cm

00.10.20.30.40.50.60.70.80.9

1

0 0.05 0.1 0.15

Bed

Exp

ansi

on (

m)

Water velocity (m/s)

Bed Expansion vs Water Velocity

Ug=0.0125m/sUg=0.025m/sUg=0.0375Ug=0.05m/sUg=0.0625m/sUg=0.075m/sUg=0.0875m/sUg=0.1m/s

0

0.2

0.4

0.6

0.8

1

1.2

0 0.02 0.04 0.06 0.08 0.1 0.12

Bed

Exp

ansi

on (

m)

Air Velocity (m/s)

Bed Expansion vs Air Velocity

Ul=0.0m/sUl=0.03m/sUl=0.06m/sUl=0.09m/sUl=0.12m/sUl=0.15m/s

39

It is observed from the above figure that for a particular velocity of water, when the air velocity

is subsequently increased there is a decrease in the bed height. This trend is more significantly

observed at lower velocities of water. Particles of dia. less than 2.58 mm show an increasing bed

expansion at higher liquid velocities. The same behavior can be observed here as the bed height

increases for water velocity 0.15m/s. The bed height also increases at liquid velocity 0.12 m/s

but only for air velocity 0.0125 m/s and 0.025 m/s and then it shows a decreasing trend as

observed in other lower cases of liquid velocity.

5.3 Phase Holdup

Individual phase holdup and their behavior with varying water and air velocity have been

discussed in this section and illustrated nicely through figures.

5.3.1 Gas Holdup

Gas hodup is obtained as mean-area weighted average of volume fraction of air at sufficient

number of points in the fluiidzed part of the bed. Since the volume fraction of air phase is not the

same at all points in the fluiidzed part of the column, hence an area weighted average of volume

fraction of air determined at regular height till the fluidized part is over. When these values are

averaged it gives the required gas holdup. Here an area weighted average of volume fraction of

air is determined at heights 10 cm, 20 cm, 30 cm etc. The following figure 15 shows the plot of

gas holdup varying with subsequent change in water velocity.

40

Fig.15. Gas holdup vs Water Velocity for initial bed height of 21.3 cm

It is observed from the plot that for a particular air velocity, the gas holdup decreases with the

increase in water velocity.

Fig.16. Gas holdup vs Air Velocity for initial bed height of 21.3 cm

Keeping the water velocity constant, it is observed that the gas holdup increases monotonically

with the increase in air velocity.

00.020.040.060.08

0.10.120.140.160.18

0.2

0 0.05 0.1 0.15

Gas

Hol

dup

Water Velocity (m/s)

Gas Holdup vs Water Velocity

Ug=0.0125m/sUg=0.025m/sUg=0.0375m/sUg=0.05m/sUg=0.0625m/sUg=0.075m/sUg=0.0875m/sUg=0.1m/s

0

0.05

0.1

0.15

0.2

0 0.025 0.05 0.075 0.1

Gas

Hol

dup

Air Velocity (m/s)

Gas Holdup vs Air Velocity

Ul=0.0m/s

Ul=0.03 m/s

Ul=0.06m/s

Ul=0.09m/s

Ul=0.12m/s

Ul=0.15m/s

41

5.3.2 Liquid Holdup

Following are the plots showing liquid holdup.

Fig.17. Liquid holdup vs Water Velocity for initial bed height of 21.3 cm

It is observed from the previous plot that liquid holdup increases with the increase in water

velocity at a given velocity of air.

Fig.18. Liquid Holdup vs Air Velocity for initial bed height of 21.3 cm

00.10.20.30.40.50.60.70.80.9

0 0.05 0.1 0.15 0.2

Liq

uid

Hol

dup

Water Velocity(m/s)

Liquid Holdup Vs Water Velocity

Ug=0.0125m/sUg=0.025m/sUg=0.0375m/sUg=0.05m/sUg=0.0625m/sUg=0.075m/sUg=0.0875m/sUg=0.1m/s

0

0.2

0.4

0.6

0.8

1

0 0.025 0.05 0.075 0.1

Liq

uid

Hol

dup

Air Velocity (m/s)

Liquid Holdup vs Air Velocity

Ul=0.0m/sUl=0.03m/sUl=0.06m/sUl=0.09m/sUl=0.12m/sUl=0.15m/s

42

From the plot in figure 19, it is observed that for a given water velocity, the liquid holdup

decreases as the inlet air velocity is increased. For water velocity lying in the range of 0.06m/s to

0.12 m/s, the decreasing trend is more in comparison to the much lower velocities.

5.3.3 Solid Holdup

It shows a trend opposite to the bed expansion. Following figures show the variation in solid

holdup with varying inlet water velocity and air velocity.

Fig.19. Solid Holdup vs Water Velocity for initial bed height of 21.3 cm

From figure 19 it is clear that the solid holdup decreases with increase in inlet water velocity.

This corresponds to the same trend in which the expanded bed height decreases with the increase

in water velocity as discussed earlier.

Next is shown the variation of solid holdup with the air velocity.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.05 0.1 0.15 0.2

Solid

Hol

dup

Water Velocity (m/s)

Solid Holdup vs Water Velocity

Ug=0.0125m/sUg=0.025m/sUg=0.0375m/sUg=0.05m/sUg=0.0625m/sUg=0.075m/sUg=0.0875m/sUg=0.1m/s

43

Fig.20. Solid Holdup vs Air Velocity for initial bed height of 21.3 cm

It is seen that solid holdup increases with increase in air velocity corresponding to water velocity

ranging from 0.0 m/s to 0.12 m/s. At higher water velocity of 0.15 m/s, solid holdup decreases as

air velocity is increased since the bed expansion takes place and hence the volume fraction of the

glass beads (solid phase) decreases.

Fig.21.Comparision of experimental result of gas holdup with that of the simulated result

obtained at inlet air velocities of 0.0625m/s for initial bed height of 21.3 cm.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.025 0.05 0.075 0.1

Solid

Hol

dup

Air Velocity (m/s)

Solid Holdup vs Air Velocity

Ul=0.0m/sUl=0.03m/sUl=0.06m/sUl=0.09m/sUl=0.12m/sUl=0.15m/s

44

The above plot shows comparison of experimental result of gas holdup with that of simulated

result obtained at air inlet velocities 0.0625 m/s for initial bed height 21.3 cm. The simulated

results have been compared with the experimental results of Jena et al (2009).It shows that

simulated results are in excellent agreement with experimental results with deviation of less than

9%. The reason for deviation may be that the glass beads used in experiment have a range of

diameters while in the simulation all glass beads are taken to be of the same diameter.

45

CHAPTER 6

CONCLUSION

6.1 Conclusion

CFD simulations of gas-liquid-solid fluidized bed were carried by employing Eulerian -Eulerian

approach for different operating conditions. In three phase fluidization the hydrodynamic

variables studied were gas, liquid and solid holdup, bed expansion, XY plots and velocity

vectors. The three phase interactions which are very complex and time consuming in

experiments have been studied in this present work. The main aim was to analyze the individual

phase holdup behavior in a three phase fluidized bed.

Main conclusions that can be drawn are:

Contour of volume fraction of water shows that the volume fraction of water is less in

fluidized part of the column in comparison to the rest part.

Gas holdup is more in fluidized part of the bed as illustrated by the contours for air.

Bed expands when liquid velocity is increased at constant air velocity as illustrated from

the plot of bed expansion vs liquid velocity. For low inlet velocity of water, the bed

height decreases with the subsequent increase in air velocity.

Trends of Gas Holdup vs inlet Air velocity shows that gas holdup increases with increase

in air velocity obtained at different inlet water velocities.

Trends of liquid holdup vs inlet Water Velocity show that liquid holdup increase with the

increase in inlet water velocity at different inlet velocities of air.

Solid holdup obtained decreases with the increase in inlet water velocity at different inlet

velocity of air.

46

REFERENCES

Bahary, M.,1994. Experimental and computational studies of Hydrodynamics in three-phase and

two- phase fluidized beds. Ph.D. thesis. Illinois Institute of Technology, Chicago.

Bakker, A.,2002. Applied computational fluid dynamics, Fluent Introductory notes. Fluent Inc.

Lebanon.

Bauer, M., Eigenberger, G., 2001.Multiscale Modeling of Hydrodynamics, Mass Transfer and

Reaction in Bubble Column Re-actors, Chemical Engineering Science 56, 1067-1074.

Comte, M.P., Bastoul, D., Hebrard, G., Roustan, M., Lazarova, V., 1997. Hydrodynamics of a

three phase fluidized bed- the inverse turbulent bed. Chemical Engineering Science 52, 3971-

3977.

Calis, H. P. A., Nijenhuis, J., Paikert, B. C., Dautzenberg, F. M., van den Bleek, C. M.,2001.

CFD Modelling and Experimental Validation of Pressure Drop and Flow Profile in a Novel

Structured Catalytic Reactor Packing, Chemical Engineering Science 56, 1713-1720.

Chaitanya,V.,Parimal N.,Ravindranath, G.,Satish, K, B.,Albert, K.,2003. Basics of computational

fluid dynamics analysis.

Epstien, N., 1981. Three phase fluidization: some knowledge gaps. Canadian Journal of

Chemical Engineering 59, 649-757.

Fluent 6.3 Documentation, 2006. Fluent Inc., Lebanon.

Fan, L.S., 1989. Gas–Liquid–Solid Fluidization Engineering, Butterworth-Heinemann, Boston

Ghoshdastidar S P., 1998.Computer simulation of flow and heat transfer. Tata Mc Graw-Hill

Publishing Company Limited, New Delhi.

Heidari, A., 2007 .Multiphase flow regimes and approaches in CFD simulations. The Chemical

Engineering journal 49,157-162.

47

Howard, J.R., 1989. “Fluidized bed Technology: Principles and applications”, Adam Higler,

New York.

Jena, H.M., 2009. Hydrodynamics of gas-liquid –solid fluidized and semi-fluidized beds., Ph.D

thesis. National Institute of Technology, Rourkela.

Joshi, J.B., Vitankar, V.S., Dhotre, M.T., 2002. A low Reynolds number k-휀 model for the

prediction of flow pattern and pressure drop in bubble column reactors. Chemical Engineering

Science 57, 3235-3250.

Kim S.D., Gabriel Wild, Joo H. Han, 1989, Phase holdup characteristics in three phase fluidized

beds. The chemical Engineering journal, 43, 67-73

Kim S.D., Jan, S., 1980, Phase holdup characteristics of three phase fluidized beds of mixed

particles, hwahak konghak 18, 375-384.

Krishnaiah, k., Guru, S., Sekar, V., 1993. Hydrodynamic studies on inverse gas-liquid-solid

fluidization. The Chemical Engineering journal 51,109-112.

Kumar, A., 2009. CFD Modeling of Gas-Liquid-Solid Fluidized Bed. B.Tech. Thesis,

NIT,Rourkela, India.

Lee S.L.P., DeLasa, H.I., 1987. Phase holdups in three-phase fluidized beds. A.I.Ch.E. Journal

33, 1359–1370.

Lee, Q, F.,2006..Fluidization:Chemical and Biochemical Engineering

Levenspiel, O., Kunii, D., 1991.“Fluidization Engineering”, 2nd edition, ISBN:0-409-90233-0.

Mahmood, A., 2008. Hydrodynamic Characteristics of a Gas- Liquid –Solid Fluidized Bed

Containing a Binary Mixture of Particles. M.S. Thesis, Chemical Engineering Department,

Ministry of higher education & Scientific Research University of Technology.

Muroyama, K., Fan, L.S., 1985. Fundamentals of Gas-Liquid-Solid Fluidization, AIChE Journal

3, 1-34.

48

Mohapatra, D., Rakh, K., 2007. CFD Modeling of Multi-phase Fluidized Bed. B. Tech. Thesis,

NIT, Rourkela, India.

Panneerselvam R., Savithri S., SurenderG.D. "CFD based investigations on hydrodynamics and

energy dissipation due to solid motion in liquid fluidized bed", Chemical Engineering Journal

132 ,159–171.

Rusche, H., 2002. Computational Fluid Dynamics of Dispersed Two-Phase Flows at High Phase

Fractions. Ph.D. thesis. Imperial College of Science, Technology & Medicine, London.

Schallenberg, J., Enb, J.H., Hempel, D.C., 2005. The important role of local dispersed phase

hold-ups for thecalculation of three-phaase bubble columns. Chemical Engineering science 60,

6027-6030.

Schubert, M., Larachi, F.,2009. Phase Holdups in Three-Phase Semifluidized Beds and the

Generalized Bubble Wake Model. Industrial and Engineering Chemistry Research 48, 8393–

8401.

Sivaguru,K., Meera Sheriffa begum, K.M., 2009. Hydrodynamic studies on three-phase fluidized

bed using CFD analysis. Chemical Engineering Journal 155, 207-214.

Sivakumar, V., Senthilkumar, K., Akilamudhan, P., 2008. Prediction of liquid holdup and solid

holdup in three-phase fluidized bed :Air/Newtonian and non-Newtonian liquid systems.

Chemical and BioChemical Engineering Journal 22(4) 401-410.

Sokol, W., HAlafani, M.R., 1999. Hydrodynamics of gas-liquid-solid fluidized bed bioreactor

with a low density biomass support. BioChemical Engineering Journal 3, 185-192.

Spalding et al., 1972 B. E. Launder and D. B. Spalding. Lectures in Mathematical Models of

Turbulence. Academic Press, London, England.

Yin, F. H., Sun, C. G., Afacan, A., Nandakumar, K., Chuang, K. T., 2000. CFD Modeling of

Mass Transfer Processes in Randomly Packed Distillation Columns, Industrial and Engineering

Chemistry Research 39, 1369 – 1380.